Petroleum cuts must be pre-treated before use to eliminate unwanted compounds and to obtain products which satisfy current regulations. In that context, gas oil cut hydrogenation is of particular importance because of new regulations regarding sulfur and aromatic compound levels imposed for that type of fuel.
Desulfurization is generally carried out under conditions and using catalysts which are not also capable of hydrogenating the aromatic compounds. Thus a second treatment has to be carried out on the cut for which the sulfur content has already been reduced, to hydrogenate the aromatic compounds contained in that cut. This step is generally effected by bringing the cut, in the presence of hydrogen, into contact with a catalyst which is generally based on a noble metal. However, since the desulfurization process can never completely eliminate the sulfur-containing and nitrogen-containing compounds, the catalysts used must be able to operate in the presence of those compounds, which are powerful inhibitors of the activity of noble metals, and they must as a consequence have good thio-resistant properties.
A group of catalysts with thio-resistant properties has already been identified. Platinum and palladium based catalysts have been described in respect of their thio-resistance. When using an alumina type support (U.S. Pat. No. 3,943,053), it is reported that the metal contents and the conditions for preparation must be precisely controlled. This constraint causes distinct problems in terms of flexibility during industrial scale-up for these formulations. The use of supports based on silica-aluminas or zeolites has also been reported. Examples are U.S. Pat. Nos. 4,960,505, 5,308,814 and 5,151,172. Those documents teach that the type of zeolite which can produce the desired properties is highly specific. Further, the use of such supports has a number of major disadvantages, among them their preparation, which includes a forming step requiring the use of mineral binders such as aluminas. The preparation methods thus must allow noble metals to be selectively deposited onto the zeolitic material as opposed to deposit onto the binder, which is an additional restriction with that type of catalyst.